Gallium nitride semiconductor devices that use nitride semiconductor are used in light emitting devices such as light emitting diode device (LED) and laser diode device (LD), light receiving devices such as solar cell and light sensor, and electronic devices such as transistor and powder devices. Semiconductor laser device that employs nitride semiconductor, in particular, is believed to be capable of oscillating in a wide range of visible light spectrum from ultraviolet to red light, and is expected to have variety of applications such as light sources for laser printer and optical network in addition to the light source for optical disk system.
The gallium nitride semiconductor device of the prior art often has pn heterojunction that combines an n-type active layer including In and p-type cladding layer including Al, as the basic structure. Since the n-type active layer including In can easily decompose, a thin cap layer made of AlGaN is often formed between the n-type active layer and the p-type cladding layer in order to prevent the active layer from decomposing when growing the p-type cladding layer at a relatively high temperature.
FIG. 3 shows a schematic sectional view of a nitride semiconductor laser as an example of the gallium nitride semiconductor device of the prior art. The nitride semiconductor laser shown in FIG. 3 has double heterojunction structure wherein an MQW active layer made of InGaN is interposed between the n-type and p-type AlGaN cladding layers. An n-type AlGaN contact layer 103, an n-type InGaN crack prevention layer 104, an n-type AlGaN/GaN super-lattice cladding layer 105, an undoped GaN optical guide layer 106, a quantum well active layer 107 made of InGaN, a p-type AlGaN cap layer 108, an undoped GaN optical guide layer 109, p-type AlGaN/GaN super-lattice cladding layer 110, and a p-type GaN contact layer 111 are stacked successively via a buffer layer 102 on a GaN substrate 101 that was grown by ELOG process. Reference numeral 162 denotes a protective film made of ZrO2, 164 denotes a multi-layer dielectric film made of SiO2 and TiO2, 120 denotes a p-type electrode, 121 denotes an n-type electrode and 122 and 123 denote lead-out electrodes.
The active layer 107 has such an MQW structure as undoped Inx1Ga1-x1N well layers (0<x1<1) and undoped Inx2Ga1-x2N barrier layers (0≦x2<1, x1>x2) are stacked alternately a required number of times. The p-type AlGaN cap layer 108, together with the active layer 107, forms a pn heterojunction so as to effectively confine electrons within the active layer 107 thereby to reduce the threshold of the laser. Also because the p-type cap layer 108 has a role to supply holes to the active layer 107, it is doped with Mg in a high concentration. The p-type cap layer 108 may be grown to a small thickness of about 15 to 500 Å, and as a thin film, it can be grown at a lower temperature than in the case of the p-type optical guide layer 109 and the p-type optical cladding layer 110. Consequently, forming the p-type cap layer 108 makes it easier to suppress the decomposition of the active layer 107 that includes In than in the case of forming the p-type optical guide layer 109 and other layers directly on the active layer.
The gallium nitride semiconductor laser having the structure shown in FIG. 3 is capable of achieving a long lifetime of more than 10,000 hours under the condition of continuous oscillation with an output power of 5 mW at the room temperature.
However, there is a demand to increase the lifetime of the gallium nitride semiconductor device in order to expand the applications thereof. For the gallium nitride semiconductor laser, in particular, it is extremely important to increase the lifetime of the device and there is also a demand to improve the threshold characteristic of operation at high temperatures.